1. Field of the Invention
This invention relates to a laser-diode-pumped solid state laser in which a solid state laser crystal is pumped by a laser diode (semiconductor laser), and more particularly to such a laser-diode-pumped solid state laser of a relatively high output type where the resonator internal power reaches 10 W or more.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a laser-diode-pumped solid state laser in which a solid state laser crystal doped with a rare earth element such as neodymium is pumped by a light beam produced by a laser diode.
Such a laser-diode-pumped solid state laser is generally provided with a wavelength selector which selects one of a plurality of oscillation modes in the resonator and makes the laser oscillate in a single longitudinal mode.
Further in the laser-diode-pumped solid state laser, the temperatures of the solid state laser crystal and the resonator are generally controlled to predetermined temperatures in order to suppress fluctuation in the oscillating wavelength. The temperature control is generally effected by placing the resonator on a cooling surface of an electronic cooling element (a Peltier element), detecting the inside temperature of the resonator and controlling the electronic cooling element on the basis of the detected inside temperature of the resonator.
Further, in the laser-diode-pumped solid state laser, the intensity of at least a part of a solid state laser beam emitted from the resonator (may be a wavelength-converted laser beam) is detected by a photodetector and the laser diode is controlled on the basis of the output signal of the photodetector so that the output of the laser diode is kept constant, thereby stabilizing the output of the solid state laser. This control is generally referred to as "an automatic power control".
Conventionally most of the laser-diode-pumped solid state lasers are of a relative low output, lower than 10 W in the resonator internal power. In such a laser-diode-pumped solid state laser, the oscillating wavelength and the output are generally stabilized by providing a wavelength selector and controlling the temperature of the resonator.
However in the recent laser-diode-pumped solid state laser which is relatively high in output power, not lower than 10 W in the internal power of the resonator, there has been a problem that the longitudinal mode hops even if a wavelength selector is provided and the temperature of the resonator is controlled. Further, in the conventional high output type laser-diode-pumped solid state laser, there have been problems that a plurality of longitudinal modes are generated for a long time upon starting of the laser and the output becomes unstable and that noise is generated due to mode competition.